Such gas turbines are typically designed as stationary gas turbines but also as aero engines which have metal casing parts which have a large thermal capacity and therefore also a high heat storage capability. In the casing parts, provision is typically made for discharge openings which are designed to direct the cooling fluid from the annular passage to further fluidically connected sections of the gas turbine for additional cooling and sealing purposes. Furthermore, the cooling fluid naturally also serves for thermal conditioning, i.e. for cooling the components which delimit the annular passage.
Depending on the operating condition of the gas turbine, it is necessary to feed the annular passage only with a comparatively small quantity of cooling fluid. This consequently leads to free convection cells being able to form in the annular passage, which convection cells are formed inside the annular passage on account of the temperature differences. The free convection cells have in turn a significant influence on the overall flow in the annular passage so that sometimes undesirable asymmetries are formed in the cooling fluid distribution in the annular passage. Therefore, increasingly local regions, which are hotter than other regions, are formed inside the annular passage. As a result of the temperature inhomogeneity in the temperature distribution, the geometries of the gas turbine can be distorted on account of the different material expansions of the individual local regions.
On account of this distortion, a loss of axial symmetry of the gas turbine can be the consequence, as a result of which the gap distances between the rotating and static components of the gas turbine over the cross-sectional inside circumference in the longitudinal direction of the gas turbine are different. So, it is known, for example, that on account of the uneven temperature distribution inside the gas turbine during the start-up process an ovalization or a warping of the entire casing can occur.
Moreover, the convective distribution of the cooling fluid in the annular passage provides for different cooling fluid temperatures over the circumference of the annular passage. As a result, however, variable cooling fluid temperatures ensue on those sections of the gas turbine on which additional cooling functions are to be performed by the cooling fluid, like in the case of turbine blade cooling which is fed from the annular passage. As a consequence, it may be required that these sections have to be designed for increased cooling fluid consumption, which is technically necessary to avoid.
Accordingly, the components in the gas turbine are always to be designed for the most unfavorable geometric case, as a result of which an efficiency loss or cost increase is to be added in.
Such problems are typically ignored in the case of most gas turbines and the efficiency losses or cost increase are tolerated. Especially tolerated is the fact that the gap distances between the inner casing part and the rotating components are larger than they might be, specifically when the gas turbine would not be subjected to distortion or warping.
Some attempts for avoiding these problems are already known from the prior art, for example from EP1505261A1, which gives instruction to subdivide the annular passage in the circumferential direction into zones which are fluidically separated from each other.
This fluidic subdivision enables the forming of free convection cells over the entire annular passage to be at least partially prevented, or even completely prevented. Comparable technical provisions are known for example from GB 2,017,826 A, U.S. Pat. No. 5,219,268 A or 4,631,913 A. A disadvantage of this fluidic subdivision of the annular passage into zones is, however, that the individual annulus sectors have to be expensively fed with cooling fluid from the outside and all the sectors require a feed line in each case. Moreover, the fluidic separation of individual sectors proves to be problematic to the extent that individual sectors may require different cooling capacity. As a consequence, different quantities of cooling fluid per time unit are to be fed to the sectors, as a result of which the expenditure on open-loop and closed-loop control is also significantly increased.